Security at a popular UK music festival stood by and watched revellers enjoy ‘laughing gas’, an investigation has revealed.

Partygoers at South West Four on Clapham Common, London, openly took the nitrous oxide – which is inhaled from a balloon and gives users a short high – in front of security staff over the weekend.

The BBC investigation found that teenage girls were lying on their backs inhaling balloons as a uniformed guard watched on and neither confiscated the drugs nor threw them out.

Asked whether laughing gas was therefore allowed in SW4, the BBC quotes the guard as saying: ‘I haven’t even got a clue mate. If they got through [entry searches], that’s not my problem. I don’t really care.

‘If a fight broke out I would call for backup, but if they’re doing that I don’t care.’